Code Switching in French Diplomatic Correspondence and Intercepted Letter of Barbé-Marbois (1782)

Encoded letters from November 1780 to April 1782 between Vergennes (French Foreign Minister), Luzerene (Wikipedia) (French Minister to the United States), his secretary Barbé-Marbois (Wikipedia), Comte de Montmorin (Wikipedia) (French Ambassador to Madrid), and Marquis of Castries (Wikipedia) (Minister of the Navy) are preserved in the British National Archives (CO 5/40). Many of these letters belong to the period when peace negotiations for American independence were about to begin after Cornwallis' surrender in October 1781.

The present author partially reconstructed the code from the interlined deciphering and identified four different codes. It is remarkable that two or three different codes were used even in the same letter. Switching of extensive diplomatic codes (as opposed to switching of substitution tables of the letters of the alphabet as, e.g., devised by Cornwallis (see another article)) shows the attention paid by the Bourbon dynasty to cryptologic security. Such code switching is not observed in the few encoded letters examined by the present author from the Napoleonic period (see another article).

(Note (27 January 2017): It has been observed that two different ciphers were often used in the same dispatch by the Spanish under Philip II. See Cp.21 under another article.)

Codes Used between Barbé-Marbois and Vergennes

Codes

Encoded letters in March to April 1782 from Barbé-Marbois to Vergennes, French Foreign Minister, use two codes ("Code A" and "Code B" herein). Both have entries up to 1200 representing letters, syllables, and words, arranged in non-alphabetical order (i.e., two-part code).

Common words are given multiple numbers. For example, "et" could be represented by 124, 622, 776, 819, 1106 in Code A and 322, 328, 401, 791, 806, 961, 995, 1002 in Code B.

The letters "i" and "j" as well as "u" and "v" were written differently but were not distinguished in the alphabet, as was typical at the time. Thus, 96 and 480 in Code A and 364 in Code B may represent either "ie" or "je". The figure 653 in Code A may represent "uves" in prouves or "vue".

As with many other French codes, variant forms may be represented by one code number. Thus, the figures 600 in Code A and 3 in Code B may represent either "peu" or "peut" and 1014 in Code B may represent either "dans" or "dant". The figures 1166 in Code A and 1177 in Code B may represent tous/tout/toute/toutes.

Code Switching

Different codes were used even in a single letter. Code switching, which occurs in the middle of a page, appears to be indicated by some nonsense code numbers. The following is the list of the letters in CO5/40 examined by the present author, showing such nonsense numbers appearing at the switching as well as the beginning and the end of the letter. The exact nature of the indication system is yet to be ascertained.

1053(Code A indicator???) 362(- in Code A) [text in Code A] 519(- in Code A) 164(- in Code A) 1049(- in Code A) 105(- in Code A) *1053 appears before the portions encoded with either Code A or Code B and thus may not be an indicator, while 362 represents "propose" in Code B in several instances and cannot indicate switching from Code B to Code A.

f.422-425, Barbé-Marbois to Vergennes, 1 April 1782

Suspicions around Berbé-Marbois' Letter of 13 March 1782

Although the French were allied with the Americans, their interests did not always match those of the Americans.

In the letter of 13 March 1782, Barbé-Marbois expressed views against the American claim for the Newfoundland fisheries and, for other fisheries, suggested that the King might give assistance but should not be committed.
The letter was captured by the British and deciphered. The English translation was given to John Jay, one of the American peace commissioners in Paris, in early September.

Jay sent a copy to Livingston on 18 September 1782 (Jay to Livingston, 18 September 1782 (RDCUS);
Jay to Livingston, 13 October 1782 (RDCUS)), saying "Let us be honest and grateful to France, but let us think for ourselves." The report gave strength to anti-French sentiments in the Continetal Congress (James Madison Papers, p.443, n.2).

The English text provided by the British includes the following passage.

it appears to me possible to use means for preventing the consequences of success to Mr. Samuel Adams and his party, and I take the liberty of submitting them to your discernment and indulgence. One of those means would be for the king to cause it to be intimated to Congress or the ministers, "His surprise that the Newfoundland fisheries have been included in the additional instructions; that the United States set forth therein pretensions without paying regard to the king's rights, and without considering the impossibility they are under of making conquests and keeping what belongs to Great Britain." His majesty might at the same time cause a promise to be given to Congress "of his assistance for procuring admission to the other fisheries, declaring, however, that he would not be answerable for the success, and that he is bound to nothing, as the treaty makes no mention of that article."Marbois to Vergennes, 13 March 1782 (RDCUS)

Comparison shows that the translation given by the British is accurate. Historians now consider it genuine (Carl Van Doren, Benjamin Franklin, Penguin Books, p.686). However, given only the English translation made by their opponents, the Americans could not take it at its face value.

It was natural for Jay, albeit inclined to negotiate with the British without consulting the French, to "endeavor to get a copy of it, in order the better to decide on the correctness of the translation." In the minister's reply (Livingston to Jay, 30 December 1782 (RDCUS)), Livingston also considered that "though true on the whole, yet it may have been colored in the translation."

Benjamin Franklin, another peace commissioner, who, unlike Jay, was inclined to believe in the French faith, thought the letter might be only a British forgery (Carl Van Doren, Benjamin Franklin, Penguin Books, p.686).

It took more than 100 years for historians to be sure of its authenticity (Ralph E. Weber, United States Diplomatic Codes and Ciphers p.85-86; At the time of the publication of RDCUS (1888), the original of the letter was not found and its editor cast a doubt on its authenticity in footnotes.).

Code Used between Vergennes, Luzerne, and Montmorin

A similar but different code (Code D herein) was used between Luzerne and Comte de Montmorin, then French ambassador to Madrid.

f.344-345, the enclosure of the above: translation of Harrison, Governor of Virginia, to Luzerne, February 1782

864(Code A indicator?) [text in Code A] 164(-) 140(-)

f.360-367, Luzerne to Vergennes, 7 March 1782, "No.223"

1191 1041(Code A indicator?) [text in Code A]

905(Code B indicator?) 572(- in Code B) [text in Code B]

755(Code A indicator?) 864(- in Code A) [text in Code A]

905(Code B indicator?) 572(- in Code B) [text in Code B] 115(null?)

Code Used between Barbé-Marbois and Marquis of Castries

A letter dated 17 March 1782 (CO5/40 f.392-399) from Barbé-Marbois to Marquis of Castries, minister of the navy, (Wikipedia) used a similar but different code ("Code C" herein). The highest numbers are 1199 "ser" and 1201 "Monsr ..." (not identified in the British deciphering). The word "et" is given 592, 593, 1194. Both "ans" and "ant" may be represented by 747.

The Count of Guichen mentioned has an entry in Wikipedia. From the absence of nonsense code numbers at the beginning, it appears code switching was not used in the correspondence with the minister of the navy.

Code C is also used in f.305-317, Barbé-Marbois to Marquis de Castries, Philadelphia, 2 February 1782 "No.4".

Sources

British National Archives: CO 5/40 "Intercepted letters to and from American colonists. (1770-1782)" (catalogue entry) Please note that the present author has seen only the beginning and the last portions of the file, i.e., ff.1-50 and ff.285-425. Of these, ff.1-50 appear to include nothing of interest for cryptology. Before f.285 is a title sheet "Part-3."

Wharton, The Revolutionary Diplomatic Correspondence of the United States, vols.1-6, (RDCUS) (1888) (The Library of Congress)

&COPY;2015 S.Tomokiyo
First posted on 11 July 2015 with a title: "Code Switching in French Diplomatic Correspondence of Barbe-Marbois (1782)". Expanded by additionally using CO 5/40 ff.285-367 and retitled on 10 January 2016. Last modified on 27 January 2017.Articles on Historical Cryptography